Credit cards are typically mailed to their users in nondescript paper envelopes. Unscrupulous persons have intercepted such envelopes before the addressees received them. An interceptor would then tamper with the envelope in an often successful attempt to determine the credit card characters contained therein. The credit card characters could then be used to fraudulently charge purchases to the addressee's account.
An interceptor can determine the characters within the envelope by applying a piece of paper to the surface of the envelope, against which surface the card characters abut, and rubbing a pencil or carbon against the paper to imprint the image of the raised characters upon the paper.
In other schemes, the envelopes are opened by clandestine means and resealed after the information has been noted.